Cutting tools comprise a main body made for example from carbide metal, cermet, steel or high-speed steel. Frequently a single-layer or multi-layer coating is applied to the main body to increase the service lives or also to improve the cutting properties. That coating includes for example metallic hard material layers, oxide layers or the like. CVD processes (chemical vapour deposition) and/or PVD processes (physical vapour deposition) are employed to apply the coating. A plurality of layers within a coating can be applied exclusively by means of CVD processes, exclusively by means of PVD processes or by a combination of those processes.
In regard to the PVD processes a distinction is drawn between various variants in such processes such as magnetron sputtering, arc vapour deposition (arc PVD), ion plating, electron beam vapour deposition and laser ablation. Magnetron sputtering and arc vapour deposition count among the PVD processes most frequently used for coating tools. Within individual PVD process variants there are in turn different modifications such as for example unpulsed or pulsed magnetron sputtering or unpulsed or pulsed arc vapour deposition.
The target in the PVD process can comprise a pure metal or a combination of two or more metals. If the target includes a plurality of metals then all those metals are simultaneously incorporated into the layer of a coating, which is built up in the PVD process.
The quantitative ratio of the metals relative to each other in the layer which is built up will depend on the quantitative ratio of the metals in the target, but also on the conditions in the PVD process as individual metals are dissolved out of the target in higher quantities under given conditions and/or are deposited on the substrate in higher quantities, than other metals. Besides the pure metals, oxidic, nitridic, carbidic forms of the metals or mixtures thereof are also used in targets.
To produce given metal compounds reactive gases are fed to the reaction chamber of the PVD process, for example nitrogen for producing nitrides, oxygen for producing oxides, carbon-bearing compounds for producing carbides, carbonitrides, oxycarbides etc. or mixtures of those gases to produce corresponding mixed compounds.
In the PVD process a so-called bias potential is generally applied to the substrates to be coated in order to achieve the surface energy necessary for the growth process, and thus atomic mobility. The energy is necessary to achieve crystalline structures in a growing layer. When applying insulating layers using the PVD process, which applies for example to a large number of metal oxide compounds, the effectively applied bias potential is reduced during the growth process with increasing layer thickness, because of the insulating properties of the layer material, and that worsens the growth conditions at the layer surface and in addition can ultimately lead to exclusively or primarily amorphous structures being grown.
Ramm, J. et al., Pulse enhanced electron emission (P3e™) arc evaporation and the synthesis of wear resistant Al—Cr—O coatings in corundum structure, Surface and Coatings Technology 202 (2007), pages 876-883, describe the deposit of aluminium oxide-chromium oxide layers by pulsed arc vapour deposition (arc PVD). The deposited layers firstly present a mixed crystal structure.
Teixeira, V. et al., Deposition of composite and nanolaminate ceramic coatings by sputtering, Vacuum 67 (2002), pages 477-483, describe the deposit of thin zirconium oxide/aluminium oxide layers in the nanometer range by magnetron sputtering. The layers exhibit crystalline components of zirconium oxide, but only amorphous components of aluminium oxide.
Trinh, D. H. et al., Radio frequency dual magnetron sputtering deposition and characterization of nanocomposite Al203 —ZrO2 thin films, J. Vac. Sc. Techn. A 24(2), March/April 2006, pages 309-316, describe the deposit of very thin zirconium oxide/aluminium oxide layers in the nanometer range by magnetron sputtering, which present crystalline components of tetragonal zirconium oxide but only amorphous components of aluminium oxide.
WO-A-2007/121954 describes the production of a hard substance layer on a substrate by means of magnetron sputtering, wherein the hard substance layer contains the metallic elements Al, Cr and Si as well as non-metallic elements from the group B, C, N and O. The atomic proportion of oxygen in the non-metallic elements is greater than 30%. The hard substance layer preferably contains crystalline phases and/or mixed phases in the system Al—Cr—Si—O. Both cubic phases of the space group Fd3m and also hexagonal phases of the space group R-3C can be formed.
EP-A-1 029 105 and EP-A-1 253 215 describe coated cutting tools for metal machining with a carbide metal, cermet or ceramic body and a hard and wear-resistant and heat-resistant coating which is deposited using the DMS (dual magnetron sputtering)-PVD method, wherein at least one layer and preferably the outermost layer comprises Al2O3 and further layers if at all present are produced between the tool body and the Al2O3 layer from metal nitrides and/or carbides of the metallic elements Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and/or Al. The Al2O3 layers comprise dense, fine-grain, crystalline γ-Al2O3 and can also include other phases from the γ-series.
Thus purely crystalline and purely amorphous systems as well as systems with crystalline grains in an amorphous matrix are known from the state of the art. The crystalline phases include binary systems or mixed crystals of known crystal systems.
X-ray and electron diffraction are used as methods of investigating metal oxide layers in order to determine the lattice plane spacings occurring in the crystal structure (d-values) and/or to demonstrate amorphous structures. In that respect electron diffraction is more advantageous in relation to X-ray diffraction because of the lower wavelength for investigating disordered grains involving grain sizes of 10-50 nm.